Broad band acoustic transducer



' Jan. 31, 1967 D. E. ANDREWS, JR 3,302,163

BROAD BAND ACOUSTIC TRANSDUCER Filed Aug. 31, 1965 INVENTOR. DAN/EL E. ANDREWS, JR

United States Patent M 3,302,163 BROAD BAND ACOUSTIC TRANSDUCER Daniel E. Andrews, Jr., 1563 Yost Drive, San Diego, Calif. 92109 Filed Aug. 31, 1965, Ser. No. 484,134 13 Claims. (Cl. 3408) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates both to transducers for converting oscillatory electrical energy into acoustic waves, and to those for converting acoustic waves into electrical energy, and, in particular, to those transducers especially adapted for underwater sound applications.

Efforts heretofore to increase the bandwidth of directional underwater acoustic transducers and especially or transducers used as sources of underwater sound, have left much to be desired. Some progress has been made through the use of piezoelectric and magnetostrictive cylinders arranged in linear arrays both with and without reflectors and through the employment of mass-loaded piezoelectric stacks in plane and curved arrays, but frequently either the bandwidth or the directional properties are not adequate. Also, the cost of the mass-loaded ele ments is often prohibitive.

It is therefore a primary object of this invention to provide an improved and relatively inexpensive transducer having a relatively broad frequency response.

Another object is to provide a transducer having an improved directional response and one that is suitable for use both singly and in arrays.

According to the present invention a hollow, generallycylindrical electromechanical transducer element is mounted concentrically between a pair of conical reflectors one of which has a relatively hard surface radially facing the cylindrical transducer element, while the other has a similarly-disposed relatively soft surface.

It is recognized that the fundamental radial mode of vibration 'of a hollow circular cylinder immersed in a fluid produces vibrations in the fluid at the inner and outer surfaces of the cylinder, the two sets of vibrations being in exact phase opposition. For example,'if an electromechanical (magnetostrictive, piezoelectric or other) cylinder is electrically driven at some frequency in the vicinity of or below that of its fundamental radial mode, movement of its inner surface will tend to rarefy the enclosed medium whenever the movement of the outer surface tends to compress the medium surrounding it, and vice versa.

As already stated, the presentinvention is capable of operating either in a receiving or a radiating mode, although the apparatus normally would be adapted for both modes. In a radiating mode, oscillatory electrical energy is supplied to the transducer element to produce the compressional and rarefactional waves which travel radially and are reflected outwardly by the conical reflectors, preferably in a direction axially aligned with the cylindrical transducer. However, the reflection-s, instead of maintaining the out-of-phase relationship, are in phase due to the relative acoustic impedance of the soft-surfaced reflector which is such as to produce a phase reversal in its reflections, as well as the impedance of the hard-surfaced reflector which is such as to preserve the phase of its impinging waves. Inasmuch as this arrangement acoustically loads both the inner and outer surfaces of the radiating cylinder, "better coupling, bandwidth and .efficiency is obtained. Also the large elfective aperture 3,302,163 Patented Jan. 31, 1967 (nominally twice the wall thickness plus four times the height of the cylinder) provides useful directional characteristics. In addition, the bandwidth is further enhanced by the boost in response at low frequencies due to a resonant-pipe effect, the effective length of the pipe being equal to twice the cylinder height plus half the wave length being considered.

In its receiving mode, in-phase incoming acoustic waves have their phase relationship reversed with the result that their energy, impinging on the cylindrical transducers produces a mechanical vibration which, in turn, produces an electrical output.

Other objects and features of this invention will become apparent to those skilled in the art by referring to the specific embodiment described in the following specifications and shown in the accompanying drawing in which the single figure comprises a half section of a round transducer body incorporating this invention.

Referring to the drawing, transducer element 10 is a hollow cylinder of electromechanically-active material which will radially expand and contract when stimulated by an electric field or will generate an electric output when subjected to a radial mechanical stress. The cylinder may be fabricated from one piece or an assembly of pieces, all or only part of which may be electromechanically active. Typically, ferroelectric ceramics, such as barium titanate and lead zirconate, and scrolls or stacks of rings made of magnetostrictive material are used although assemblies made of crystals of Rochelle salt amonium-dihydrogen-phosphate, or quartz or other electromechanically active substances can be employed. In the example shown, element 10 is a radially polarized ferroelectric ceramic cylinder, the cylindrical surfaces of which are covered with fired-on silver electrodes, 11 and 12. Insulated electrical leads 13 are electrically connected by soldering or other suitable means to these electrodes. When an alternating voltage is applied to the leads and to the electrodes 11 and 12 the diameter of the cylinder 10 alternately increases and decreases in rhythm with the applied wave. Conversely, when a soundwave impinges upon the sides of the cylinder so as to create time-varying stresses in the material, a corresponding electric signal, which can be measured across the terminals 13A, is generated.

Two conical reflecting surfaces 20 and 21 are disposed opposite inner and outer radiating surfaces 11A and 12A of the transducing element 10. According to an important feature of this invention, one reflecting surface presents a very high acoustic impedance while the other reflecting surface presents a very low one. It is immaterial whether the high impedance surface is inside or outside the cylindrical transducing element. In the specific example illustrated, a high impedance right'circular conical reflector 20 having an apex angle of and a height equal to the height of the cylinder 10 is mounted coaxially within the cylinder. The apex falls in or close to a plane through the top rim of the cylinder. Mounted coaxially outside the cylinder is a low impedance, truncated 90 right circular reflector 21, the smaller inner circle of which coincides with the outer lower rim of the cylinder.

Preferably, the upper rim of cone 21 extends beyond the top rim of the cylinder 10. The inner surface of the conical member 21 is made to have a very low acoustic impedance 'by covering it with a soft pressure release material 21A having high compliance at the power and frequencies of operation. The acoustically soft material may be a corkor gas-impregnated elastomer such as the commercially available materials known as Corprene or Celtite rubber. The material is of substantial thickness and is preferably backed by a supporting structural conical member 21B. Conical structure 2113 conveniently supports inner cone and the cylinder 10, and it, in turn, is supported in an appropriate housing 22. A plate 23 is bolted across the end of the housing and contains a water-tight gland 24 used to eifect the seal between the case in a lead-in cable 13.

As the cylinder vibrates in its radial mode in response to the application of the alternating electrical power, the cylinder goes through alternate expansions and contractions. When the cylinder contracts, a radial compressional wave is radiated inwardly toward the inner reflecting surface 20 While a rarefractional cylindrical wave is radiated radially outward toward the outer reflector. On striking the inner reflector the cylindrical wave is converted by reflection into a plane wave traveling outward along the axis of the cone and the time phase of the pressure wave is preserved on its reflection from the high impedance surface. On striking the outer reflector 21, the outgoing cylindrical wave is converted by reflection into a plane wave also traveling outward along the axis of the cone, but with the phase of the outgoing disturbance being rcversed by 180". Because of this phase reversal and the overall geometry of the system both reflected waves arrive at the mouth of the tranducer at the same time and in phase. The in-phase addition of the two reflections maintains irrespective of the frequency and amplitude of the excursions of the vibrating cylinder as long as the specific acoustic impedance of the two reflecting surfaces are respectively many times greater and many times less than that of the propagating medium or, in other words, the ambient environs of the transducer. Line 30, which is perpendicular to the face of, and is the axis of symmetry for, the transducer assembly, is the direction of maximum respouse for outgoing or incoming acoustical signals. The in-phase additions cause the transducer to perform much as though it were a shaded, piston-type radiator. The annular spaces surrounding the cylindrical element may be filled with oil or a suitable potting compound, transparent to acoustical energy, or the electrical parts may be insulated by coating them with an insulating material and the annular spaces exposed to the surrounding fluid.

It will be recognized that the foregoing description has been principally with regard to a transducer adapted to convert oscillatory electrical energy into an outgoing acoustic wave. However, as has been stated, the transducer easily is adapted for the conversion of acoustic wave energy into an electrical signal. In this latter event, the acoustic wave arrives at the reflectors in phase and then the phase reversal of the relatively soft reflector produces an out-of-phase relationship which mechanically vibrates the cylinder to produce the electrical signal.

The structural parts of the transducer assembly preferably are made of corrosion resistant metals such as copper, brass or stainless steel. Although corkor gasfilled rubber-like materials are usually used for the pressure release layer 21, assemblies of compliant tubes, gasfilled bladders, and other pads and pressure-release devices known to the art may be used.

Modifications may be made in the proportions or relative dispositions of the assembly without departing from the scope of the appended claims.

What is claimed is:

1. Acoustically responsive apparatus comprising;

an electromechanical transducer element having a hollow generally-cylindrical shape;

a conical reflector disposed concentrically and exteriorally of said cylindrical element;

a second conical reflector disposed concentrically and interiorally of said cylindrical element, the two conical reflectors being disposed to reflect waves in one direction parallel to the axis of said cylindrical element and being equally spaced from the opposed surfaces of said cylindrical element so that the distances in wave lengths along any radial line to corresponding incremental areas on the conical reflector surfaces are substantially equal;

one of said conical reflectors having a relatively hard and noncompliant surface radially facing said cylindrical element and the other of said reflectors having a relatively soft and compliant surface radially facing said cylindrical element;

said relatively hard surface having an acoustic impedance sufliciently higher than its operative ambient environs for substantially preserving the phase of impinging acoustic waves and the other surface having an acoustic impedance sufliciently lower than said environs for substantially reversing the phase of impinging waves.

2. The apparatus of claim 1 wherein said second conical reflector is formed as a right circular cone of a height approximately equal to the height of said cylindrical transducer element.

3. The apparatus of claim 2 wherein said cylindrical element has a height approximately equal to one half of its inside diameter.

4. The apparatus of claim 3 wherein said exteriorallydisposed conical reflector is formed as a truncated right circular cone having an apex angle of 90 and having its smaller diameter approximately equal to the outer diameter of said cylindrical transducer element.

5. The apparatus of claim 1 wherein said exteriorallydisposed reflector is provided with said soft and compliant surface.

6. Acoustically responsive apparatus comprising;

an electromechanical transducer element having a hollow generally-cylindrical shape;

electrical means operatively coupled to said cylindrical element for producing an alternating radial motion of the cylinder whereby radially-directed compressional and rarefractional acoustic waves generated from the inner and outer surfaces of said cylindrical element are substantially out of phase;

a conical reflector disposed concentrically and exteriorally of said cylindrical element;

a second conical reflector disposed concentrically and interiorally of said cylindrical element, the two conical reflectors 'being disposed to reflect said acoustical waves in one direction parallel to the axis of the cylindrical element and being equally spaced from the opposed surfaces of the cylindrical element so that the distances in wave lengths along any radial line to corresponding incremental areas on the conical reflector surfaces are substantially equal;

one of said conical reflectors having a relatively hard and noncompliant surface radially facing said cylindrical element and the other of said reflectors having a relatively soft and compliant surface radially facing said cylindrical element;

said relatively hard surface having an acoustic impedance sufliciently higher than its operative ambient environs for substantially preserving the phase of impinging acoustic waves and the other surface having an acoustic impedance sufficiently lower than said environs for substantially reversing the phase of impinging waves;

whereby said acoustic waves reflected from said surfaces are effectively in phase in said one directional parallel to said axis.

7. The apparatus of claim 6 wherein said second conical reflector is formed as a 90 right circular cone of a height approximately equal to the height of said cylindrical transducer element.

8. The apparatus of claim 7 wherein said cylindrical element has a height approximately equal to one half of its inside diameter.

9. The apparatus of claim 8 wherein said exteriorallydisposed conical reflector is formed as a truncated right References Cited by the Examiner UNITED STATES PATENTS 2,005,741 6/ 1935 Hayes 34016 2,746,026 5/ 1956 Camp 340-8 References Cited by the Applicant UNITED STATES PATENTS 3,027,964 6/ 1958 Spragins.

10 CHEST-ER L. JUSTUS, Examiner.

I. P. MORRIS, Assistant Examiner. 

1. ACOUSTICALLY RESPONSIVE APPARATUS COMPRISING: AN ELECTROMECHANICAL TRANSDUCER ELEMENT HAVING A HOLLOW GENERALLY-CYLINDRICAL SHAPE; A CONICAL REFLECTOR DISPOSED CONCENTRICALLY AND EXTERIORALLY OF SAID CYLINDRICAL ELEMENT; A SECOND CONICAL REFLECTOR DISPOSED CONCENTRICALLY AND INTERIRALY OF SAID CYLINDRICAL ELMENT, THE TWO CONICAL REFLECTORS BEING DISPOSED TO REFLECT WAVES IN ONE DIRECTION PARALLEL TO THE AXIS OF SAID CYLINDRICAL ELEMENT AND BEING EQUALLY SPACED FROM THE OPPOSED SURFACES OF SAID CYLINDRICAL ELEMENT SO THAT THE DISTANCES IN WAVE LENGTHS ALONG ANY RADIAL LINE TO CORRESPONDING INCREMENTAL AREAS ON THE CONICAL REFLECTOR SURFACES ARE SUBSTANTIALLY EQUAL; ONE OF SAID CONICAL REFLECTORS HAVING A RELATIVELY HARD AND NONCOMPLIANT SURFACE RADIALLY FACING SAID CYLINDRICAL ELEMENT AND THE OTHER OF SAID REFLECTORS HAVING A RELATIVELY SOFT AND COMPLIANT SURFACE RADIALLY FACING SAID CYLINDRICAL ELEMENT; SAID RELATIVELY HARD SURFACE HAVING AN ACOUSTIC IMPEDANCE SUFFICIENTLY HIGHER THAN ITS OPERATIVE AMBIENT ENVIRONS FOR SUBSTANTIALLY PRESERVING THE PHASE OF IMPINGING ACOUSTIC WAVES AND THE OTHER SURFACE HAVING AN ACOUSTIC IMPEDANCE SUFFICIENTLY LOWER THAN SAID ENVIRONS FOR SUBSTANTIALLY REVERSING THE PHASE OF IMPINGING WAVES. 